Partially hydrogenated oils, the primary dietary source of the fats, have been shown to raise "bad" cholesterol. Reducing the use of trans fats could prevent 20,000 heart attacks and 7,000 deaths from heart disease a year, the FDA said.

"While consumption of potentially harmful artificial trans fat has declined over the last two decades in the United States, current intake remains a significant public health concern," FDA Commissioner Margaret Hamburg said.

Public health advocates welcomed the move.

"Artificial trans fat is a uniquely powerful promoter of heart disease, and today's announcement will hasten its eventual disappearance from the food supply," said Michael Jacobson, executive director of the advocacy group Center for Science in the Public Interest.

The FDA's proposal is not the first public effort to ban trans fats. New York City banned the use of trans fats in restaurants, including their use for deep-frying foods, and many restaurants and fast food chains, including McDonald's Corp., have eliminated their use.

Some European countries have also taken steps. Denmark, Switzerland and Iceland regulate the sale of many foods containing trans fats.

Products that still contain trans fats include some varieties of crackers, refrigerated dough, coffee creamers and ready-to-use frosting, among others. Some products will be harder to reformulate than others, FDA officials said.

"We know that technically this is not an insoluble problem," Hamburg told reporters on a conference call, adding that the use of trans fats has declined dramatically since 2006, when the agency required that trans fat levels be disclosed on package labels.

According to the Grocery Manufacturers Association, food manufacturers have voluntarily lowered the amounts of trans fats in their food products by more than 73 percent since 2005, in part by reformulating products. The FDA said the average daily intake of trans fats by Americans fell from 4.6 grams a day in 2003 to 1 gram in 2012.

A view shows the U.S. Food and Drug Administration (FDA) logo at the lobby of its headquarters in Si …

"Trans fats that are not naturally occurring have been drastically reduced," the Grocery Manufacturers said. "We look forward to working with the FDA to better understand their concerns and how our industry can better serve consumers."

It was unclear which companies would be hit hardest, or what the total cost will be, but many products well known to U.S. consumers are likely to be affected.

Among products singled out by the Center for Science in the Public Interest were various Marie Callender's pies, made by ConAgra Foods Inc; Diamond Foods' Pop Secret microwave popcorn; and cinnamon rolls from Pillsbury Co., owned by General Mills.

Richard Galloway, president of industry consulting firm Galloway and Associates and a 25-year veteran of the soy-processing industry, said switching formulations is costly and time consuming.

In general, "food companies take about two years from the time they are introduced to an alternative ingredient until they can commit to a switchover," Galloway said.

Hydrogenation is a chemical process that converts liquid vegetable oils into solid or semi-solid fats. Partially hydrogenated oils extend the shelf life of foods, and certain types of popcorn, fish sticks, pies, donuts and pizza depend on trans fats for their taste and texture.

Coming up with alternative recipes for products that contain trans fats will largely be a matter of trial and error, industry experts say. Palm kernel oil, which is solid at room temperature and has become a popular substitute for trans fats, might work in some cases but some products might have to be dropped.

"If this rule becomes final the impact to companies will include the cost of finding an alternative to trans fats," said Justin Prochnow, a lawyer with Greenberg Traurig LLP who advises food companies on FDA-related matters.

At the other end of the food chain, the American Soybean Association said that seed and technology companies have developed soybean varieties rich in high oleic fatty acids that eliminate the need for partial hydrogenation. "Significant quantities" of such high oleic soybean oil should be in the marketplace by 2016, the group said.

The FDA's proposal is subject to a 60-day public comment period in which food companies are expected to outline how long they expect it to take them to reformulate products.

If the proposal becomes final, partially hydrogenated oils would be considered food additives and would not be allowed in food unless authorized by health regulators. The ruling would not affect trans fat that occur naturally in small amounts in certain meat and dairy products.

Companies wishing to include trans fats in their products would have to meet the safety standards applied to food additives and prove with reasonable certainty that they do not cause harm.

"GENERALLY RECOGNIZED AS SAFE"

It has been more than half a century since U.S. regulations governing food additives were last revised. In that time, the number of chemicals in the food supply has risen from fewer than 2,000 to an estimated 10,000, many of which are never reviewed by the FDA.

Under loose regulations created more than 50 years ago to help companies avoid lengthy delays in getting food additives approved, the FDA created a list of products considered "generally recognized as safe" (GRAS).

Companies can either petition to get their ingredients affirmed safe by the FDA, or they can declare them safe based on their own research or that of hired consultants. The FDA has the option to challenge such declarations.

The FDA's Hamburg said in an interview on Thursday that while the GRAS system provides the current legal framework for regulating food additives, the system bears re-examining to see if it is adequate to ensure the safety of the food supply.

"We do need to be thinking about what is needed to update laws and processes," she said.

The agency is already under pressure to ban the use of caffeine in energy drinks. Caffeine was long ago declared to be a GRAS product in cola-type drinks. Yet the agency has not challenged companies to prove the safety of caffeine in other products or other beverages.

"Caffeine is one we are looking at very seriously," Hamburg said, adding that the agency hosted a major meeting of experts over the summer under the auspices of the independent Institute of Medicine. "It's an ongoing process but one in which we are deeply engaged."